Traditional methods of reducing the diameter of cylindrical bodies include "die necking" in which one end of the cylindrical body is forced into a conical die which exerts a compressive force to reduce the diameter and "roll necking" in which a roll is engaged with the exterior of the cylindrical body as it is rotated to generate the shoulder profile by a spinning or beading process.
U.S. Pat. No. 3,995,572 describes a method and apparatus for producing a seamless can body with a reduced diameter opening for receiving an aerosol valve. Starting with a cylindrical workpiece a truncated conical portion supporting a cylindrical portion of reduced diameter is formed by each of a sequence of dies to finally form a shoulder characterized by the curvilinear configuration imposed by each die. The disadvantage of this series of "die necking" operations is that each die only brings about a relatively small reduction in can diameter so that the expense of several press tools is incurred. Furthermore, the shoulder produced has a corrugated or stepped shape which is not always desirable.
Beverage cans are now well known in which the top of the side wall is necked in to receive an aluminium can end of diameter smaller than the outside diameter of the bulk of the can body. The objective in such cans is to use less aluminium can end material, thus the present invention also seeks to provide a method of making neck portions of reduced diameter.
These prior art cans are usually 2.585" (65.6 mm) diameter necked down to 2.462 (62.5 mm) diameter and made by one of several roll forming methods currently available.
In one known roll forming method, as described in British Pat. No. 1,330,346 (U.S. Pat. No. 3,688,538) a peripheral edge margin of the sidewall of the can body is spun on a mandrel as an external roll compresses the margin to generate a shoulder neck and flange. However, a collapsible mandrel is required to carry out this method. In another roll forming method, as described in our British Pat. No. 1,534,716, (U.S. Pat. No. 4,058,998) the can body is supported in axial compression while a marginal portion adjacent the open end of the body is deformed radially inwards by a pair of external rollers so that the combination of axial and radial forces generate a shoulder, neck and flange. This method requires only a simple solid chuck, because the neck is formed into free space.